Science of penis-barb shaving

A paper in Royal Society Biology Letter by University of Toronto biologist Lucia Kwan describes the strange, adversarial clawed sex-organs of some guppies. Kwan experimented with shaving the barbs off of the penises of some male guppies to investigate the relative advantages of claws for mating with "unreceptive females." She concluded that the claws were a "sexually antagonistic trait" that evolved to allow males to force females to mate with them.

This strongly suggests that the claws are a “sexually antagonistic trait”—one that benefits one sex over the other. In this case, they help the males to grasp resistant females. If they were simply for anchoring sperm, the de-clawed males should suffer when mating with all females, rather than just the unreceptive ones.

This is the latest in a small but growing line of phenotypic engineering genital-shaving studies, which aim to work out just why animal sex organs are so bizarrely adorned. One group laser-shaved the spikes from a fly’s penis to show that they’re like biological Velcro, allowing males to latch onto females. Another team did the same thing with a seed beetle’s penis to show that its terrifying spikes aren’t anchors—their role seems to be to puncture the female’s genital tract for reasons best known to the seed beetle.

The penises of these insects are just as varied in shape, size and spikiness as those of the guppies, so these experiments suggest that the battle of the sexes has fuelled the evolution of these groups, helping them to diversify into the many species we see today. To understand that process, scientists would need to compare the organs of many different species, but these shaving experiments are certainly a good first step.